I think I've never posted on the Grim forums, and I love Grim Fandango (so very much).
And sorry if this comes a bit late, but I wanted to give my points on why it wouldn't be such a good idea to make a sequel (or prequel....or a spin off for that matter).
I totally agree that sequels are not necesarily bad (I like more Toy Story 2 than the first one. And Home Alone 2? Simply brilliant). Anyway, a Grim sequel beign a bad idea has nothing to do with the sequels issue (from my point of view).
It has a lot more to do with the fact of trying to continue or "expanding" a story that was conceived in such a way that would be ruined if exploded in those ways.
I think one of the greatest things Tim Schafer and his team achieved is the sense of mystery and uncertainty mood it's weaven beneath the story. We, for example, know very little about everybody's life when they where in the world of the living. We don't know what Manny has done in his life to be paying it in the land of the dead. And I think that's cool, not knowing that allows you (the player) to try to imagine how his previous life was and such things. Also, not knowing that is perhaps a way of driving your attention to what matters most in the story, the events that occured, the corruption and all of this subjects on which you can think about.
So, for me, the story begins and finishes in the perfect moment. And trying to explore the afters and befores with other than our own imaginations...well...I think it rather spoils things.
And basically that's why I think it wouldn't be such a good idea. Not to mention that Tim wouldn't be involved....and god only knows how an unexperienced writer and designer could ruin a brilliant story.